Graphene represents an atomically thin layer of carbon in which the carbon atoms reside within a single sheet or a few stacked sheets (e.g., about 20 or less) of fused six-membered rings forming an extended planar lattice of interconnected hexagonal molecules, although the planar lattice need not necessarily contain six-membered rings exclusively. In this context, graphene represents a planar arrangement of sp2- and sp-hybridized carbon atoms that may or may not exhibit a long range crystalline order. In its various forms, graphene has garnered widespread interest for use in a number of applications, primarily due to its favorable combination of high electrical and thermal conductivity values, good in-plane mechanical strength, and unique optical and electronic properties. In many aspects, the properties of graphene parallel those of carbon nanotubes, since both nanomaterials are based upon an extended and electrically conjugated carbon framework. Other two-dimensional materials having an extended planar structure are also of interest for various applications. As used herein, the term “two-dimensional material” will refer to any extended planar structure of atomic thickness, including both single- and multi-layer variants thereof. Multi-layer two-dimensional materials can include up to about 20 stacked layers.
Because of its extended planar structure, graphene offers several features that are not shared with carbon nanotubes. Of particular interest to industry are large-area graphene films for applications such as, for example, special barrier layers, coatings, large area conductive elements (e.g., RF radiators or antennas), integrated circuits, transparent electrodes, solar cells, gas barriers, flexible electronics and the like. In addition, graphene films can be produced in bulk much more inexpensively at the present time than can carbon nanotubes.
Large-area graphene films of atomic thicknesses and containing single- or multi-layer graphene can be produced by a variety of chemical vapor deposition (CVD) processes. CVD growth takes place on a metal-containing growth substrate, such as a copper or nickel foil, and the graphene is strongly adhered to the growth substrate following synthesis. Even the outer graphene layers in multi-layer graphene, which are spatially separated from the surface of the growth substrate, can remain strongly adhered to the growth substrate. The strong adherence of graphene to its growth substrate can make intact removal of the graphene film difficult.
Metal growth substrates are often undesirable for use in downstream applications utilizing a graphene film. For example, chemical, electrical, or functional incompatibility can result when attempting to utilize a graphene film that is still adhered to or in contact with its metal growth substrate. Accordingly, it can often be desirable to transfer a graphene film from its metal growth substrate onto a secondary substrate, also referred to herein as a “functional substrate” or a “receiving substrate.” The secondary substrate can exhibit properties that are better suited to meet the needs of a particular application.
Removing a graphene film from its growth substrate and subsequently transferring the graphene film to a secondary substrate can be difficult for a number of reasons. Although graphene has high mechanical strength on an atomic basis, it can be fairly fragile on the macroscale once it has been removed from its growth substrate. For example, tearing, fracturing and/or buckling can occur in the process of liberating a graphene film from its growth substrate. Tearing and buckling can produce poor surface conformality and coverage upon transferring the graphene film to a secondary substrate. Some processes for affecting removal of a graphene film from its growth substrate can also produce undesirable chemical damage to the graphene film, which can degrade its desirable properties.
One solution for addressing the difficulties posed by unsupported graphene films involves depositing a supporting layer on the graphene film that temporarily provides mechanical stabilization during the transfer process. Poly(methyl methacrylate) (PMMA) layers have been used in this regard. Once transfer to the secondary substrate is complete, the supporting layer is removed from the graphene film, meaning that the supporting layer is sacrificial and does not remain associated with the graphene film in its end deployment configuration. The use of a sacrificial supporting layer to promote transfer of graphene films can be undesirable for a number of reasons including, for example, incomplete layer removal following transfer, chemical damage to the graphene film and/or the secondary substrate during the layer removal process, poor surface conformality of the graphene film to the secondary substrate due to constrainment by the supporting layer, and potential incursion of the supporting layer into perforations within the graphene film. Chemicals used to affect removal of the sacrificial supporting layer can often be particularly incompatible with the polymer materials forming the secondary substrate. Further, the additional processing operations needed to deposit and then remove the sacrificial supporting layer can be undesirable from a time and cost standpoint.
In view of the foregoing, facile techniques for manipulating graphene films without using a sacrificial supporting layer would be of considerable benefit in the art. The present disclosure satisfies the foregoing need and provides related advantages as well.